1. Field of the Invention
The present invention relates to a thermal head, and more particularly, to a thermal head capable of half-tone printing.
2. Description of the Related Art
Thermal heads with a novel faculty have been intensively developed of late such that half-tone printing can be effected by changing the size of printing dots to be printed. Such thermal heads are disclosed in "Half Tone Wax Transfer Using a Novel Thermal Head", THE FOURTH INTERNATIONAL CONGRESS ON ADVANCES IN NON-IMPACT PRINTING TECHNOLOGIES pp. 273-276, "Thermo-Convergent Ink-Transfer Printing (TCIP) for Full Color Reproduction", Proceedings of 2nd Non-impact Printing Technologies Symposium pp. 105-108, "Published Unexamined Japanese Patent Application Nos. 60-58877 and 60-78768". Each of the thermal heads is provided with a number of heating resistors each having a narrow-width portion. Electric current flowing through each heating resistor increases its density at the narrow-width portion, so that heat is produced from a local region in the high-density portion. In thermal heads, only those regions which produce heat higher than a certain value are effective for printing, and the regions capable of generating sufficient heat for the printing spread in proportion to voltage applied to the heating resistors. If higher voltage is applied to the heating resistors, therefore, the size of the printing dots increases in proportion.
In the conventional thermal head of this type, however, the heating resistors have a complicated configuration, so that manufacturing them requires much time and labor, and it is difficult to provide uniform properties for the numerous heating resistors.